Countrymouse is right. Any nurse or therapist I’ve ever talked to has said the best way to treat pressure sores is to prevent them, meaning take the pressure off that part of the body. Have Dad lay on his side in bed for a few hours. Then he can get back in bed for a few hours. Pressure sores can cause deadly infections, and once you have one, they are very difficult to heal.
But to be really honest, the only way round this is mobilisation - changing your father's position at least every hour. Is that possible?
What kind of chair is he using?